Martian alluvial fans: Comparing fan and catchment areas as a proxy for paleoclimate

Alluvial fans, formed by fluvial processes or mass wasting events, have been used to interpret paleoclimate, most often using the ratio of fan area to the area of its catchment that provides water and sediment. The recent discovery of alluvial fans on Mars contributes to evidence of previous water flow and one or more major climate transitions that led to the planet’s current hyper-arid climate. A recently compiled database (Morgan et al., 2022) cataloged 1,501 alluvial fans globally, and showed a systematic difference in the scaling relationship between fan area and catchment area for cases in impact craters compared to those outside craters. This study tests the hypotheses that 1) scaling relationships between fan and catchment areas vary systematically with crater age and latitude; and 2) observed differences in fan-catchment scaling relationships for craters versus non-craters arise due to differences in basin geometry and catchment relief. To test these hypotheses, I analyzed subsets of alluvial fans and catchments within the global database and fit power-law relationships in the form A_fan=c_1 〖A_catchment〗^n. This analysis revealed that 424 alluvial fans in non-craters in the dichotomy boundary (nfans = 102) and southern highlands (nfans = 211), as well as in Noachian-aged craters (nfans = 111), display low exponent values suggesting a weak relationship between catchment area and fan area. In contrast, 1,177 fans formed during the Hesperian (nfans = 83) and Amazonian (nfans = 25) periods, and in fans hosted within craters (nfans = 1,069), exhibit high exponents that suggest a more direct relationship between fan area and catchment area. Subsequently, I considered 5 pairs of study sites with the same catchment area to assess controls on their differing fan areas. The approach for inferring basin geometry using topographic profiles failed for 3/10 cases, suggesting that an alternative geophysical approach would be needed. However, for these case studies, catchment relief was lower for fans in craters, which may explain why fan scaling relationships between fan area and catchment area are consistently different for craters versus non-craters at a global scale. These analyses suggest that the age of the fan, geographic location, and the geomorphic setting (crater versus non-crater) are important context for interpreting the scaling relationship between fan area and catchment area on Mars. To place further constraints on the Martian paleoclimate, future research is necessary to discern whether differences in fan geometry were the result of geologic conditions (e.g., basin geometry) versus differences in climate.